1. Field of the Invention
This invention relates to semiconductor fabrication, and more particularly to a method for fabricating a photomask, which has a pattern including a narrow line width and a small pitch between lines.
2. Description of Related Art
In a photolithography process, photomask is the main tool to transfer a circuit pattern onto a wafer so that the photomask plays an essential role in semiconductor fabrication. The main part of the photomask is a transparent substrate and is planar. A circuit pattern is formed by an opaque layer, which includes a desired pattern to cover a portion of the transparent substrate. When the photomask is exposed to a light source, a circuit pattern is projected onto the wafer. As the integration of a device increases, the photomask needs a narrower line width to satisfy the reduced device dimension.
FIG. 1A and FIG. 1B are cross-sectional views of a photomask, schematically illustrating a conventional fabrication process of the photomask. In FIG. 1A, a typical photomask includes a transparent substrate 10, which is made of glass or quartz. An anti-reflection layer 12 is formed over the transparent substrate 10. An opaque layer 14 is formed over the anti-reflection layer 12 by chemical vapor deposition (CVD). The opaque layer 14 includes, for example, chromium, ion oxide, silicon nitride, or aluminum. The light transmission of the opaque layer 14 is about 10%-35%. In FIG. 1B, according to a desired pattern, the opaque layer 14 is patterned to form a transparent region 18 and an opaque region 16. A photomask is thereby formed.
However, since the transparent region 18 and the opaque region 16 are formed by photolithography and etching, their spatial resolution is limited by equipment capability of photolithography. This causes the line with of pattern to be limited and not to be reduced. This is a problem for semiconductor fabrication in the next generation, which has greater integration.